1. Field of the Invention
The present invention relates to a low-resistance ITO thin film and a method for manufacturing such a thin film, and more particularly, to a low-resistance ITO thin film and method for manufacturing such a film used for a transparent electrode in a solar battery, display device such as liquid crystal display (LCD) and the like.
2. Description of the Related Art
As a transparent electrode material, ITO (Indium Tin Oxide), FTO (Fluorine doped Tin Oxide) and AZO (Aluminum doped Zinc Oxide) have been used. Among these materials, ITO has been mainly used in a liquid crystal display, and FTO has been mainly used in a solar battery.
As liquid crystal displays become larger and finer, i.e., higher resolution, there is a demand to reduce the electrical resistance of ITO. For example, in a STN (Super Twisted Nematic) liquid crystal display, the transparent electrode works, i.e., functions as a signal electrode, and has a strip shape. As the display device increases in size, the strip becomes longer and as the display device becomes finer, i.e., increased resolution, the strip becomes thinner. As a result, the electrical resistance along the length of the strip from one end to the other increases, which drops or decreases the electrical voltage therebetween and makes it difficult for proper switching of the liquid crystal molecules.
In a TFT (Thin Film Transistor) liquid crystal display, the signal electrode has been ordinarily made of metallic material. However, in order to simplify the production steps and reduce production cost by simplification of the device structure, the transparent electrode has been used as the signal electrode. In this display also, as the display device increases in size and resolution, the electrical resistance from one end to the other of the strip electrode increases. For the above reason, the transparent electrode is currently used as a signal electrode only in a display having a diagonal size of 11 inches.
In a solar battery, it is important to improve the efficiency-of the battery. Factors that contribute to the efficiency include (1) confining the light energy emitted in a photoelectric material, (2) improvement of collecting effective photoelectric carriers and contribution to the photovoltaic effect thereof, (3) reduced loss due to the recombined photoelectric carriers, (4) reduced loss of series resistance, (5) reduced loss of voltage factor and (6) collecting a wider range spectrum of light energy. The electrical resistance of the transparent electrode is a series resistance loss, which influences a conversion efficiency particularly in a large size element. For the above reason, the reduced resistance of the transparent electrode is also required in a solar battery.
With recent rapid developments of the liquid crystal displays, there have been some experiments to reduce the electrical resistance of ITO. For example, Ishibashi et al. succeeded in manufacturing ITO film having a resistivity of 1.5 xc3x9710xe2x88x924 xcexa9cm on a substrate having a temperature of 200xc2x0 C. by reducing plasma impedance to form a thin film with low sputtering voltage using a DC sputtering method (S. Ishibashi, Y. Higuchi, Y. Ota, and K. Nakamura, J. VaC. Sci. Technol. A8(1990)1403). Also, it is reported that when the plasma impedance is reduced by increasing the magnetic field intensity from 140G to 480G, the sputtering voltage can be reduced from 540V to 330V with a constant discharge current, which makes it possible to reduce the resistance of the electrode (Y. Shigesato, S. Takaki, and T. Haranoh, J. Appl. Phys., 71(7)(1992)3356). When the sputtering is conducted at a high voltage, the thin film is grown with high-energy particle irradiation so that a high degree of uniform strain and compression pressure are introduced into the thin film. On the contrary, when the sputtering is conducted with a low voltage, the number of high-energy particles is reduced. As a result, the crystallization of the ITO is improved and thereby increases carrier density by reducing damage to the crystal structure which traps the donor and decreases density of neutral scattering center to improve the mobility thereof, which enables a reduction of the resistance of the film (Y. Shigesato, xe2x80x9cTechnology for Transparent Conductive Filmxe2x80x9d, P112, Ohmusha, 1999).
Yamada et al. developed a vapor deposition device using a high current oxygen cluster ion beam and succeeded in manufacturing an ITO thin film having a resistivity of 8.4xc3x9710xe2x88x925 xcexa9cm by using the above device. Though production of a film having a resistivity lower than 1xc3x9710xe2x88x925 xcexa9cm has been previously reported a number of times so far, this report is the first repeatable process for manufacturing such a film. In the above vapor deposition method, oxygen is introduced to the growing thin film by irradiating oxygen cluster ions to produce a thin film oxide. In this technology, for example, the oxygen cluster ions having an average cluster size of 1000 is accelerated at 10 keV and then irradiated to the substrate. Since the kinetic energy of each oxygen molecule is as low as 10 eV, a high quality ITO film with low resistance is obtained without damage to the crystal structure.
Japanese Laid-Open Patent Application No. 7-262829 discloses a method for manufacturing a thin film by using xenon (Xe) or krypton (Kr) gas instead of argon (Ar) gas in a direct-current sputtering, high-frequency sputtering or ion beam sputtering method to obtain an ITO thin film having a resistivity lower than 1xc3x9710xe2x88x924 xcexa9cm.
The low-voltage sputtering method has an advantage in that CI damage to the thin film can be reduced since the number of high-energy particles is reduced. However, since the high-energy particles cannot be completely removed in the process, C) a film having a resistivity lower than 1xc3x9710xe2x88x924 xcexa9cm cannot be produced.
The high-current cluster ion beam vapor deposition method has an advantage in that a film having a resistivity lower than 1xc3x9710xe2x88x924 xcexa9cm can be produced without damage to the crystal structure since the kinetic energy of each oxygen particle is low. However, by this method, a large-scale film production cannot be practically realized.
In the method disclosed in Japanese Laid-Open Patent Application No. 7-262829, since the sputtering method is utilized, it has a disadvantage in that it is not clean and the targets are attached to a wall of the chamber which will cause dust. Also, since the oxygen pressure cannot be set high, composition-controlling characteristics are limited. Further, since the process cannot be conducted in a high vacuum atmosphere, a condition requiring a low level or substantially zero level of oxygen cannot be effectively realized. Moreover, since the thickness of the thin film cannot be controlled, production of a thin film in an atomic layer growth mode cannot be carried out.
It is therefore an object of the present invention to By provide a low-resistance thin film having a resistivity on the order of, or lower than 10xe2x88x924 xcexa9cm, and a method for manufacturing such a film.
In order to solve the above-mentioned problem, there is provided a method for manufacturing a low-resistance ITO film comprising a step of depositing an ITO film on a crystalline substrate by a pulsed laser vapor deposition method at a temperature of 500-1000xc2x0 C.
According to the above invention, a low-resistance ITO thin film having a resistivity lower than 1xc3x9710xe2x88x924 xcexa9cm is provided. The pulsed laser vapor deposition method has advantages in that it is clean, oxygen pressure can be set high and the thickness of the film can be properly controlled. Therefore, by using the method, a low-resistance ITO thin film excellent in resistance, mobility, and carrier density, having an improved crystal structure and uniform thickness can be provided. In the above method, the thin film may be manufactured on a large scale by rotating the substrate, arranging a plurality of laser beam or the like.
Also, there is provided a method for manufacturing a low-resistance ITO film comprising a step of depositing an ITO film on a crystalline substrate by one of low-voltage sputtering, oxygen cluster beam deposition, chemical vapor deposition, metal organic chemical vapor deposition, metal organic chemical vapor depositionxe2x80x94atomic layer deposition, and molecule beam epitaxy.
According to the above invention, a low-resistance ITO thin film having a resistivity lower than 1xc3x9710xe2x88x924 xcexa9cm is provided. The method has an advantage in that ITO thin film can be produced in a large scale at lower cost.
FIG. 1 is a graph showing a relationship between a temperature of the substrate and a conductivity (Scmxe2x88x921) of the ITO film according to an example of the present invention.